The formation of contact holes or vias is a critical process during the fabrication of semiconductor devices. Metal wiring in a device forms vertical and horizontal interconnections that are shrinking in size and increasing in complexity as technology advances to 100 nm ground rules and beyond. Current contact interconnect technology that has a 130 to 150 nm minimum feature size is typically based on a tungsten (W) plug that is formed by a chemical vapor deposition (CVD) comprising H2 and WF6 to produce a W layer that has good step coverage on a substrate. However, a Ti/TiN barrier layer is usually required to improve W adhesion to the substrate. Furthermore, a W nucleation layer is necessary to improve the W deposition process. Therefore, at least three deposition steps are employed in plug formation which adds complexity and cost to the device fabrication.
As depicted in FIG. 1a, a conventional plug formation process typically involves depositing a dielectric layer 3 on a substrate 1 containing a conductive layer 2. A contact hole 4 is patterned in dielectric layer 3. Next, a conformal Ti/TiN barrier layer 5 is deposited on dielectric layer 3 and within contact hole 4 by a CVD or plasma enhanced CVD (PECVD) method. A W nucleation layer 6 is then grown on the barrier layer 5 followed by CVD or PECVD deposition of a W layer 7 on nucleation layer 6.
Referring to FIG. 1b, a planarization step such as a chemical mechanical polish (CMP) process is used to lower the level of layers 5, 6, 7 so they are coplanar with the top of contact hole 4. In the case of a contact hole 4 that has a width w of about 100 nm or less, the conventional plug formation process does not adequately fill the contact hole 4. Note that an opening 8 still exists in contact hole 4 and this space is considered a defect that will degrade the performance of the final device. Therefore, an improved method is needed to fill a contact hole having a width of about 100 nm or less. Ideally, the most cost effective solution is a single material that can be deposited to completely fill the contact hole with good step coverage and without forming any voids.
A metal plug comprised of TiN is described in U.S. Pat. No. 5,998,871. The TiN plug is used as an interconnect from a polysilicon electrode to an underlying conductive layer and is placed between Ti silicide layers that serve to reduce the resistance in the device. The method of TiN deposition is not specified.
A TiN contact that is deposited by a one or two step CVD or plasma enhanced CVD process is mentioned in U.S. Pat. No. 6,037,252. A two step method is necessary to achieve a 100% conformal layer on the substrate. While this technique is successful for filling contacts with a diameter in the 130 to 150 nm range, a problem similar to that depicted in FIG. 1 is anticipated for filling holes that have a width of 100 nm or less.
Atomic layer deposition (ALD) is a newer approach to filling contacts that involves depositing a monolayer of precursor on a substrate, purging the chamber, and introducing a reactant that reacts with the precursor to leave a monolayer of product. The cycle is typically repeated many times to build a layer with a sufficient thickness to be functional. For example, in U.S. Pat. No. 6,203,613, a metal nitrate precursor such as Ti(NO3)4 is reduced with NH3 to yield a 5 nm thick film of TiN after 167 cycles. A slightly modified ALD technique is described in U.S. Pat. No. 6,270,572 in which a precursor is injected twice to enable a more complete coverage of a substrate before a reactant is introduced into the ALD chamber. The reactant is then purged and reinjected to provide a precise stoichiometric composition. In this case a TiN film is grown at a rate of about 1 Angstrom per cycle using TiCl4 as precursor and NH3 as reactant. Using the same concept, a TiSiN thin film is formed by an ALD method in U.S. Pat. No. 6,468,924. Here, three different steps are used to introduce a Ti source gas, a N source gas and a Si source gas with a purge gas incorporated between each of the reactant gas pulses.
Since residual chloride is a contamination issue, an alternative means of introducing Ti into an ALD chamber is desirable. Although nitrates avoid the chloride contamination concern, they are highly flammable and explosive. Therefore, a Ti compound that is safer to handle than Ti(NO3)4 and which forms gaseous by-products is needed for an improved ALD method for filling small openings. In order to provide versatility in an interconnect scheme, alternative barrier layers containing a metal and nitrogen are needed. In some cases, a multi-element barrier layer could provide an advantage.